The present invention relates to a borehole opener, particularly for enlarging this hole underneath a casing in the field of oil prospecting, the hole opener comprising a body of longitudinal axis, a duct for drilling fluid, formed longitudinally in the body, and at least two hole-opening arms which have an active part equipped with cutting means, which are distributed symmetrically in the body about the longitudinal axis, and which are arranged therein in such a way that they can be moved between a position of rest in the body and an active position partially out of the body.
Prospecting for oil of natural gas, for example, requires the probing of increasingly deep depths. What is more, it is desirable to drill with small diameters so as to reduce the drilling time and tooling costs. The counterpart to this is an increase in pressure drop for the drilling fluid flowing between the wall of the hole and the drill string and therefore a reduction in the ability to remove particles of formation as a result of the lack of circulation of the drilling fluid at a sufficiently high flow rate. As a result, control over the equivalent density of the sludge formed by the drilling fluid and these particles of formation can rapidly be lost. This equivalent density gives rise to a pressure which may be desirable at certain formations, in order to somewhat shore up the bored wall, and is therefore usually monitored to make sure that this pressure does not exceed a value above which at least this bored wall is excessively contaminated with the sludge and/or is excessively damaged in terms of its texture and composition and/or may even be destroyed.
In order to avoid these problems with the bored wall, it is also necessary to be able easily to adjust this equivalent density. This then entails improving the circulation of the sludge so as to encourage the removal of particles of formation which would build up in the hole and which would have an adverse influence on the equivalent density.
To achieve this it is known practice possibly to make use of a conventional hole opener, with moving arms, which can enlarge the diameter of the hole already bored to up to twice its original size. Hole openers known to date comprise mechanisms, of the hinge and pivot-pin type, which are rather delicate to be used at a high rate of advance and are too long to be placed between the drill bit and, in particular, an angled coupling well known to those skilled in the art and employed for directional drilling.
Drill bits with two offset longitudinal axes (bi-center bits) and pilot tools (pilot bits) associated with boring tools and comprising a hole-opening fin fixedly projecting from the rest of the bit are already known. These devices are able to open out the hole at the same time as boring it. However, because of their asymmetric structure, these devices have an imbalanced behavior and because of this produce vibrational forces which are detrimental to the mechanisms to which they are coupled and to the condition of the bored hole. What is more, because of this, these devices frequently cause unacceptable deviations in the bored holes.
The object of the present invention is to overcome the problems set out hereinabove and to propose a hole opener with moving arms which is compact and robust and the mechanism of which involves a very small number of moving parts, these advantageously having to be easy to remove and replace in the event of an incident occurring during operation. What is more, the hole opener of the invention is to have a small bulk in the lengthwise direction, so that it can readily be installed very near a drill bit, in a drill string, between the drill bit and the aforementioned angled coupling.
Underlying the invention, prior research was carried out into the amount of opening needed to solve the problem, explained hereinabove, of removing the sludge. Surprisingly, it was found that opening the hole out to a diameter approximately 10 to 20% greater, for example, than that of the casing or the nominal diameter of the associated drill bit, was sufficient to achieve this.
The problem of simplifying the mechanism was solved, according to the invention, by choosing that, inorder to move it from the position of rest into the active position, each hole-opening arm has a face, internal to the body, designed to be subjected directly, in the same way as an active face of a piston, to the pressure of the drilling fluid flowing through the body.
According to one embodiment of the invention, each arm is kept in the position of rest in the body, prior to a hole-opening operation, by at least one pin designed to break when the pressure of the drilling fluid flowing through the body exceeds a predetermined value higher than a maximum usual boring value.
Operations of troubleshooting, removing and replacing any problematical parts are simplified by choosing to mount the arm in the body by means of an intermediate support which acts as a housing for the arm in the body and which is fixed to the latter. As a preference, the intermediate support, the arm, elastic means for returning the arm into the body and the pin constitute an assembly designed to be assembled in advance outside the body and then installed therein.
Other details and particular features of the invention will become apparent from the appended claims and from the description of the hole opener of the invention, which description is given hereinbelow by way of nonlimiting example with reference to the appended drawings.